Broadband light emitting device lamps for providing white light output

ABSTRACT

A light emitting device (LED) includes a broadband LED chip having a multi-quantum well active region including alternating active and barrier layers. The active layers respectively include different thicknesses and/or different relative concentrations of at least two elements of a semiconductor compound, and are respectively configured to emit light of different emission wavelengths that define an asymmetric spectral distribution over a wavelength range within a visible spectrum. Related devices are also discussed.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/371,226, filed Feb. 13, 2009, which claims priority from U.S.Provisional Patent Application Ser. No. 61/029,093 filed Feb. 15, 2008,the disclosures of which are hereby incorporated by reference herein intheir entireties.

FIELD OF THE INVENTION

The present invention relates to semiconductor light emitting devices,and more particularly, to lamps including semiconductor light emittingdevices.

BACKGROUND OF THE INVENTION

Light emitting diodes and laser diodes are well known solid statelighting elements capable of generating light upon application of asufficient current. Light emitting diodes and laser diodes may begenerally referred to as light emitting devices (“LEDs”). Light emittingdevices generally include a p-n junction formed in an epitaxial layergrown on a substrate such as sapphire, silicon, silicon carbide, galliumarsenide and the like. The wavelength distribution of the lightgenerated by the LED generally depends on the material from which thep-n junction is fabricated and the structure of the thin epitaxiallayers that make up the active region of the device.

Typically, an LED chip includes a substrate, an n-type epitaxial regionformed on the substrate and a p-type epitaxial region formed on then-type epitaxial region (or vice-versa). In order to facilitate theapplication of a current to the device, an anode contact may be formedon a p-type region of the device (typically, an exposed p-type epitaxiallayer) and a cathode contact may be formed on an n-type region of thedevice (such as the substrate or an exposed n-type epitaxial layer).When a potential is applied to the ohmic contacts, electrons may beinjected into an active region from the n-type layer and holes may beinjected into the active region from the p-type layer. The radiativerecombination of electrons and holes within the active region generateslight. Some LED chips include an active region with multiple lightemitting regions or active layers (also known as multi-quantum-wellstructures) between or near the junction of the n-type and p-typelayers.

LEDs may be used in lighting/illumination applications, for example, asa replacement for conventional incandescent and/or fluorescent lighting.As such, it is often desirable to provide a lighting source thatgenerates white light having a relatively high color rendering index(CRI), so that objects illuminated by the lighting may appear morenatural. The color rendering index of a light source is an objectivemeasure of the ability of the light generated by the source toaccurately illuminate a broad range of colors. The color rendering indexranges from essentially zero for monochromatic sources to nearly 100 forincandescent sources. Alternatively, it may be desirable to provide alight source that may differ significantly from a light source with ahigh CRI index, but may still require a tailored spectrum.

In addition, the chromaticity of a particular light source may bereferred to as the “color point” of the source. For a white lightsource, the chromaticity may be referred to as the “white point” of thesource. The white point of a white light source may fall along a locusof chromaticity points corresponding to the color of light emitted by ablack-body radiator heated to a given temperature. Accordingly, a whitepoint may be identified by a correlated color temperature (CCT) of thelight source, which is the temperature at which the heated black-bodyradiator matches the color or hue of the white light source. White lighttypically has a CCT of between about 4000 and 8000K. White light with aCCT of 4000 has a yellowish color. White light with a CCT of 8000K ismore bluish in color, and may be referred to as “cool white”. “Warmwhite” may be used to describe white light with a CCT of between about2600K and 6000K, which is more reddish in color.

The light from a single-color LED may be used to provide white light bysurrounding the LED with a wavelength conversion material, such as aphosphor. The term “phosphor” may be used herein to refer to anymaterials that absorb light in one wavelength range and re-emit light ina different wavelength range, regardless of the delay between absorptionand re-emission and regardless of the wavelengths involved. A fractionof the light may also pass through the phosphor and/or be reemitted fromthe phosphor at essentially the same wavelength as the incident light,experiencing little or no down-conversion. Accordingly, the term“phosphor” may be used herein to refer to materials that are sometimescalled fluorescent and/or phosphorescent. In general, phosphors absorblight having shorter wavelengths and re-emit light having longerwavelengths. As such, some or all of the light emitted by the LED at afirst wavelength may be absorbed by the phosphor particles, which mayresponsively emit light at a second wavelength.

For example, a single blue emitting LED may be surrounded with a yellowphosphor, such as cerium-doped yttrium aluminum garnet (YAG). Theresulting light, which is a combination of blue light and yellow light,may appear white to an observer. More particularly, to produce yellowlight, a blue photon of approximately 2.66 electron volts (eV) may beabsorbed by the yellow phosphor, and a yellow photon of approximately2.11 eV may be emitted. As such, an average energy of about 0.55 eV maybe presumably lost through nonradiative processes. Thus, a blue LEDsurrounded by a yellow phosphor may lose an appreciable amount of energythrough the conversion of blue to yellow. Also, if a red phosphor isincluded to improve the color rendering, the energy loss may be evengreater, resulting in even greater reduced efficiency.

In addition, the light emitted by multiple different-colored LEDs may becombined to produce a desired intensity and/or color of white light. Forexample, when red-, green- and blue-emitting LEDs are energizedsimultaneously, the resulting combined light may appear white, or nearlywhite, depending on the relative intensities of the component red, greenand blue sources. While it may be possible to achieve fairly highluminous efficacy with such lamps (due at least in part to the lack ofphosphor conversion), color rendering may be poor due to the limitedspectral distribution of light emitted from each LED.

SUMMARY OF THE INVENTION

A multi-chip light emitting device (LED) lamp for providing white lightaccording to some embodiments of the invention includes first and secondbroadband LED chips. The first broadband LED chip includes amulti-quantum well active region including a first plurality ofalternating active and barrier layers. The first plurality of activelayers respectively include different relative concentrations of atleast two elements of a first semiconductor compound, and arerespectively configured to emit light of a plurality of differentemission wavelengths over a first wavelength range. The second broadbandLED chip includes a multi-quantum well active region including a secondplurality of alternating active and barrier layers. The second pluralityof active layers respectively include different relative concentrationsof at least two elements of a second semiconductor compound, and arerespectively configured to emit light of a plurality of differentemission wavelengths over a second wavelength range that includeswavelengths greater than those of the first wavelength range.

In some embodiments, a spectral distribution of the light emitted by atleast one of the first and second broadband LED chips may have a fullwidth at half maximum (FWHM) of greater than about 35 nanometers (nm).

In other embodiments, the light emitted by the first broadband LED chipmay define a first spectral distribution over the first wavelengthrange, and the light emitted by the second broadband LED chip may definea second spectral distribution over the second wavelength range. Aseparation between respective center wavelengths of the first and secondspectral distributions may not be greater than a sum of respective halfwidth at half maximum values of the first and second spectraldistributions.

In some embodiments, the lamp may further include a third broadband LEDchip. The third broadband LED chip may include a multi-quantum wellactive region including a third plurality of alternating active andbarrier layers. The third plurality of active layers may respectivelyinclude different relative concentrations of at least two elements of athird semiconductor compound, and may be respectively configured to emitlight of a plurality of different emission wavelengths over a thirdwavelength range that includes wavelengths greater than those of thesecond wavelength range. The third semiconductor compound may emit lightwith a third spectral distribution and the light emitted by the first,second, and third LED chips may combine to provide the appearance ofwhite light.

In other embodiments, the light emitted by the first broadband LED chipmay define a first spectral distribution over the first wavelengthrange, the light emitted by the second broadband LED chip may define asecond spectral distribution over the second wavelength range, and thelight emitted by the third broadband LED chip defines a third spectraldistribution over the third wavelength range. A separation betweenrespective center wavelengths of the first and second spectraldistributions may not be greater than a sum of respective half width athalf maximum values of the first and second spectral distributions. Aseparation between respective center wavelengths of the second and thirdspectral distributions may not be greater than a sum of respective halfwidth at half maximum values of the second and third spectraldistributions.

In some embodiments, the light emitted by the first broadband LED chipmay define a first spectral distribution over the first wavelengthrange, and the light emitted by the second broadband LED chip may definea second spectral distribution over the second wavelength range. Aseparation between respective center wavelengths of the first and secondspectral distributions may not be less than a sum of respective halfwidth at half maximum values of the first and second spectraldistributions.

In other embodiments, the lamp may further include a light conversionmaterial configured to absorb at least some of the light emitted by thefirst and/or second LED chips and re-emit light of a plurality ofdifferent emission wavelengths over a third wavelength range between thefirst and second wavelength ranges. As such, the light emitted by thefirst and second LED chips and the light conversion material may combineto provide the white light.

According to other embodiments of the present invention, a multi-chiplight emitting device (LED) lamp for providing white light includesblue, green, and red broadband LED chips. The blue broadband LED chipincludes a multi-quantum well active region comprising a first pluralityof alternating active and barrier layers, the first plurality of activelayers respectively including different relative concentrations of atleast two elements of a first semiconductor compound and respectivelyconfigured to emit light of a plurality of different emissionwavelengths over a blue wavelength range. The green broadband LED chipincludes a multi-quantum well active region including a second pluralityof alternating active and barrier layers, the second plurality of activelayers respectively including different relative concentrations of atleast two elements of a second semiconductor compound and respectivelyconfigured to emit light of a plurality of different emissionwavelengths over a green wavelength range. The red broadband LED chipincludes a multi-quantum well active region including a third pluralityof alternating active and barrier layers, the third plurality of activelayers respectively including different relative concentrations of atleast two elements of a third semiconductor compound and respectivelyconfigured to emit light of a plurality of different emissionwavelengths over a red wavelength range. The third semiconductorcompound has a narrower bandgap than the second semiconductor compound,and the second semiconductor compound has a narrower bandgap than thefirst semiconductor compound. The light emitted by the blue, green, andred broadband LED chips combines to provide an appearance of white lightwith good color rendering.

According to further embodiments of the present invention, a multi-chiplight emitting device (LED) lamp for providing white light includes blueand red broadband LED chips and a light conversion material. The bluebroadband LED chip includes a multi-quantum well active region includinga first plurality of alternating active and barrier layers. The firstplurality of active layers respectively include different relativeconcentrations of at least two elements of a first semiconductorcompound and are respectively configured to emit light of a plurality ofdifferent emission wavelengths over a blue wavelength range. The redbroadband LED chip includes a multi-quantum well active region includinga second plurality of alternating active and barrier layers. The secondplurality of active layers respectively include different relativeconcentrations of at least two elements of a second semiconductorcompound and are respectively configured to emit light of a plurality ofdifferent emission wavelengths over a red wavelength range. The lightconversion material is configured to absorb at least some of the lightemitted by the blue and/or red LED chips and re-emit light over a greenwavelength range. The second semiconductor compound has a narrowerbandgap than the first semiconductor compound. The light emitted by theblue and red LED chips and the light conversion material combines toprovide an appearance of white light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view illustrating a LED lamp according to someembodiments of the present invention.

FIG. 1B is a cross-sectional view illustrating a LED chip for use in LEDlamps according to some embodiments of the present invention.

FIG. 1C is a plan view illustrating an LED lamp according to otherembodiments of the present invention.

FIGS. 2A-2C are cross-sectional views and corresponding energy diagramsillustrating multi-quantum well structures in LED lamps according tosome embodiments of the present invention.

FIGS. 3A-3C are cross-sectional views and corresponding energy diagramsillustrating multi-quantum well structures in LED lamps according toother embodiments of the present invention.

FIGS. 4A-4D are graphs illustrating example spectral emissioncharacteristics of light emitting device lamps according to someembodiments of the present invention.

FIG. 5A is a top view illustrating a light emitting device lampaccording to further embodiments of the present invention.

FIG. 5B is a graph illustrating example spectral emissioncharacteristics of light emitting device lamps according to furtherembodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention now will be described more fully with reference tothe accompanying drawings, in which embodiments of the invention areshown. This invention may, however, be embodied in many different formsand should not be construed as limited to the embodiments set forthherein. Rather, these embodiments are provided so that this disclosurewill be thorough and complete, and will fully convey the scope of theinvention to those skilled in the art. In the drawings, the size andrelative sizes of layers and regions may be exaggerated for clarity.Like numbers refer to like elements throughout.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” another element, it can bedirectly on the other element or intervening elements may also bepresent. It will be understood that if part of an element, such as asurface, is referred to as “inner,” it is farther from the outside ofthe device than other parts of the element. Furthermore, relative termssuch as “beneath” or “overlies” may be used herein to describe arelationship of one layer or region to another layer or region relativeto a substrate or base layer as illustrated in the figures. It will beunderstood that these terms are intended to encompass differentorientations of the device in addition to the orientation depicted inthe figures. Finally, the term “directly” means that there are nointervening elements. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items.

It will also be understood that, although the terms first, second, etc.may be used herein to describe various elements, components, regions,layers and/or sections, these elements, components, regions, layersand/or sections should not be limited by these terms. These terms areonly used to distinguish one element, component, region, layer orsection from another region, layer or section. Thus, a first element,component, region, layer or section discussed below could be termed asecond element, component, region, layer or section without departingfrom the teachings of the present invention.

Embodiments of the invention are described herein with reference tocross-sectional, perspective, and/or plan view illustrations that areschematic illustrations of idealized embodiments of the invention. Assuch, variations from the shapes of the illustrations as a result, forexample, of manufacturing techniques and/or tolerances, are to beexpected. Thus, embodiments of the invention should not be construed aslimited to the particular shapes of regions illustrated herein but areto include deviations in shapes that result; for example, frommanufacturing. For example, a region illustrated or described as arectangle will, typically, have rounded or curved features due to normalmanufacturing tolerances. Thus, the regions illustrated in the figuresare schematic in nature and their shapes are not intended to illustratethe precise shape of a region of a device and are not intended to limitthe scope of the invention.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andthis specification and will not be interpreted in an idealized or overlyformal sense unless expressly so defined herein.

As used herein, the term “semiconductor light emitting device” and/or“LED” may include a light emitting diode, laser diode and/or othersemiconductor device which includes one or more semiconductor layers,which may include silicon, silicon carbide, nitride compounds, and/orother semiconductor materials. Examples of nitride compounds may includeGaN, AlN, InN, Al_(0.1)Ga_(0.9)N, Al_(0.2)In_(0.1)Ga_(0.7)N andIn_(0.1)Ga_(0.9)N. More generally, the notation (Al, In, Ga)N is usedhereinafter to refer to a nitride compound Al_(x)In_(y)Ga_(1-x-y)N,where 0≦x≦1, 0≦y≦1, and x+y≦1. A light emitting device may or may notinclude a substrate such as a sapphire, silicon, silicon carbide,germanium, gallium nitride and/or another microelectronic substrates. Alight emitting device may include one or more contact layers which mayinclude metal and/or other conductive layers. In some embodiments,ultraviolet, blue, cyan, green, amber, and/or red LEDs may be provided.The design and fabrication of semiconductor light emitting devices arewell known to those having skill in the art and need not be described indetail herein.

As used herein, the term “full width at half maximum” may refer to thewidth (in nanometers) of a spectral distribution at about half of itsmaximum value. Likewise, as used herein, the term “half width at halfmaximum” may refer to a value corresponding to half of the width (innanometers) of a distribution at half of its maximum value.

Some embodiments of the present invention may arise from a realizationthat, in conventional LED lamps including red, green, and blue LEDs, thespectral power distributions of the component LEDs may be relativelynarrow. Commercial LEDs typically have a narrow full width at halfmaximum (FWHM): red LEDs may have a FWHM of 17-18 nm, yellow LEDs mayhave a FWHM of 12-15 nm, blue LEDs may have a FWHM of 18-20 nm, andgreen LEDs may have a FWHM of 35-36 nm. The larger than typical FWHMfound in the commercial green LED may be a consequence of thedifficulties arising from controlling quantity and agglomeration ofindium (In) in the green LED quantum wells. Because of the narrowspectral width of these LEDs, objects illuminated by such light may notappear to have natural coloring due to the limited spectrum of thelight, even when multiple colors such as red, green and blue areemployed.

Accordingly, some embodiments of the present invention provide LED lampsincluding a plurality of “broadband” LED chips (also referred to hereinas “broadband LEDs”) having respective spectral outputs of greater thanabout 20 nm over red, blue, violet, yellow and amber wavelength ranges,and spectral outputs of greater than about 35 nm over the greenwavelength range. More specifically, some embodiments of the presentinvention provide LED lamps including three broadband LED chips havingtailored spectral outputs to provide improved color rendering. In someembodiments, one or more of the broadband LED chips may have a spectraldistribution with a full width at half maximum (FWHM) of greater thanabout 35 nanometers (nm). The materials and/or stoichiometries of thebroadband LEDs may be selected to provide white light output that iscomparable in brightness, performance, CRI and/or overall spectraldistribution to that of conventional light sources such as incandescentbulbs, with greater energy efficiency.

FIG. 1A illustrates an LED lamp according to some embodiments of thepresent invention. Referring now to FIG. 1A, a multi-chip LED lamp 100includes a common substrate or submount 101 including first, second, andthird die mounting regions 102 a, 102 b, and 102 c. The die mountingregions 102 a, 102 b, and 102 c are each configured to accept abroadband LED chip. As used herein, a “broadband LED” or “broadband LEDchip” refers to an LED chip configured to emit light with a spectralwidth that is broad compared to a conventional LED, i.e., greater thanabout 30-35 nm for green LEDs and greater than about 20 nm for red,blue, violet, yellow, amber, and other colored LEDs. For example, thered or blue or amber LED may emit with a spectral distribution that iscentered or has a central wavelength on red or blue or amber(respectively), but the respective spectral distributions may have afull width at half maximum of 20 nm or 25 nm or 30 nm or 40 nm or 50 nmor 75 nm or greater. Similarly, the green LED may emit with a spectraldistribution that has a full width centered on green, but with a fullwidth at half maximum of 35 nm or 40 nm or 50 nm or 75 nm or greater.The light emitted by a broadband LED may have a spectral shape thatdiffers appreciably from a Gaussian-like peak shape typically observedin the light output of conventional LEDs. The spectrum may, for example,have a “top-hat” or substantially uniform distribution, a LaPlace-shapeddistribution, a bimodal distribution, a sawtooth distribution, amultimodal distribution, and/or a distribution that is composite of morethan one of these distributions (including the Gaussian). In someembodiments of the present invention, a broadband LED may be configuredto emit light over a wavelength range of greater than about 50 nm. Inother embodiments, a broadband LED may be configured to emit light overa wavelength range of greater than about 75-100 nm.

Still referring to FIG. 1A, first, second, and third broadband LED chips103 a, 103 b, and 103 c are mounted on the die mounting regions 102 a,102 b, and 102 c of the submount 101, respectively. For example, thefirst broadband LED chip 103 a may be a blue LED chip configured to emitlight in a blue wavelength range (i.e., between about 410-495 nm), thesecond LED chip 103 b may be a green LED chip configured to emit lightin a green wavelength range (i.e., between about 495-590 nm), and thethird LED chip 103 c may be a red LED chip configured to emit light in ared wavelength range (i.e., between about 600-720 nm). As such, thelight emitted by the first, second, and third broadband LED chips 103 a,103 b, and 103 c combines to provide white light. The first, second, andthird broadband LED chips 103 a, 103 b, and 103 c may be formed ofdifferent materials that are selected to provide relatively high-CRIwhite light output at a relatively high efficiency. For example, thegreen LED chip 103 b may be formed of a semiconductor compound having anarrower bandgap than that of the semiconductor compound used in theblue LED chip 103 a, and the red LED chip 103 c may be formed of asemiconductor compound having a narrower bandgap than that of thesemiconductor compound used in the green LED chip 103 b. In addition, itshould be noted that the LED lamp 100 does not include a lightconversion material, such as a phosphor. Accordingly, because nophosphor is used, the LED lamp 100 may not involve energy lossesassociated with absorption, remission, and/or nonradiativerecombination.

Although described above with reference to particular colors of lightemitted by the LED chips 103 a, 103 b, and 103 c, it is to be understoodthat other combinations of different-colored broadband LED chips may beused to provide the white light output. For example, in someembodiments, the LED chips 103 a, 103 b, and 103 c may be cyan, yellow,and magenta LED chips.

FIG. 1B is a cross-sectional view illustrating a broadband LED chipaccording to some embodiments of the present invention. As shown in FIG.1B, the LED chip 103 includes an active region 105 sandwiched between ap-type cladding layer 108 and an n-type cladding layer 109. The LED chip103 also includes a p-type contact layer 111 on the p-type claddinglayer 108, and an n-type contact layer 112 on the n-type cladding layer109. The contact layers 111 and/or 112 may be a doped semiconductor thatis distinct from the cladding layer, which may act to spread chargebefore the cladding layer. The active region 105 is a multi-quantum wellstructure including a plurality of alternating active layers 106 and106′ and barrier layers 107, 107′, and 107″. The active layers 106 and106′ each include different stoichiometries or relative concentrationsof the elements of a semiconductor compound, and as such, are eachconfigured to emit light of different emission wavelengths over aselected wavelength range. For example, where the LED chip 103 is a blueLED chip, the active layers 106 and 106′ may be gallium nitride (GaN)layers, and each layer may include different concentrations of galliumand/or nitride configured to emit light at different wavelengths over ablue wavelength range (e.g., about 410-510 nm). Similarly, where the LEDchip 103 is a green LED chip, the active layers 106 and 106′ may beindium gallium nitride (InGaN) layers, each including differentconcentrations of indium, gallium, and/or nitride configured to emitlight at different wavelengths over a green wavelength range (e.g.,about 495-590 nm). Likewise, where the LED chip 103 is a red LED chip,the active layers 106 and 106′ may be aluminum gallium indium phosphide(AlGaInP) layers, each including different concentrations of aluminum,gallium, indium, and/or phosphide configured to emit light at differentwavelengths over a red wavelength range (e.g., about 600-720 nm).Additionally or alternatively, the active layers 106 and 106′ may beformed to different thicknesses to provide the emission wavelengths inthe desired wavelength range.

Accordingly, the active layers 106 and 106′ may have differentthicknesses and/or compositions selected to define a plurality ofdifferent bandgap energies. The stoichiometry may change not onlylayer-to-layer, but also within a particular layer. As such, when apotential is applied to ohmic contacts 126 and 128, the radiativerecombination of electrons and holes in each of the active layers 106and 106′ provides light emission at different wavelengths. In otherwords, the stoichiometries and/or widths of the layers 106 a and 106 a′may be adjusted to achieve a desired spectral output. The LED chip 103may further include a substrate 110, one or more capping layers (notshown) between the cladding layers 108 and 109 and the contacts and 126and 128, and/or one or more confinement layers (not shown) between thelast quantum well layers 107″ and 107 of the multi-quantum well activeregion 105 and the cladding layers 108 and/or 109. For example, eachconfinement layer may have a uniform or graded semiconductor alloycomposition configured to provide a transition between that of theadjacent cladding layer and the active region 105. In some embodiments,the confinement layers may provide bandgap energies between that of thecladding layers 109 and 108 and barrier layers 107 and 107″,respectively, for confining carriers (i.e., electrons and holes) topromote more efficient recombination within the active region 105. Also,the stoichiometries of the cladding layers 108 and/or 109 may be alteredto reduce differences in bandgap energies between the adjacent barrierlayers 107″ and 107.

Although illustrated in FIG. 1B with reference to particular numbers oflayers in the active region 105, it is to be understood that the numberof layers, thicknesses, and/or compositions of the layers of the activeregion 105 may be altered for different applications. For example,although illustrated as including only two active layers 106 and 106′,it is to be understood that broadband LED chips according to someembodiments of the present invention may include additional activelayers having similar and/or different stoichiometries, and the number,thickness, and/or compositions of these layers may be selected toprovide a desired spectral output. In other words, the spectral outputof the broadband LEDs may be tuned by adjusting the properties of thequantum wells of the active region 105. Because of the width of thespectral output of each broadband LED may be about 100 nm or less, thevariations in stoichiometry may be manufactured with relatively highefficiency. Further, it is to be understood that the barrier layers 107and/or 107″ may be incorporated into the adjacent cladding layers 109and/or 108, respectively, in some embodiments.

FIG. 1C illustrates an LED lamp according to other embodiments of thepresent invention and illustrates some ways in which the LEDs in thelamp may be changed to facilitate the formation of the desired lightspectral distribution. In particular, FIG. 1C shows that an LED lampincluding broadband LEDs according to some embodiments of the presentinvention may use more than one LED of a particular color, may use LEDchips that differ in size, and/or may use LED chips having differentshapes. Referring now to FIG. 1C, a multi-chip LED lamp 150 includes acommon substrate or submount 151 including first, second, third andfourth die mounting regions 152 a, 152 b, 152 c and 152 d. The diemounting regions 152 a, 152 b, 152 c and 152 d are each configured toaccept a broadband LED chip.

Still referring to FIG. 1C, first, second, third and fourth broadbandLED chips 153 a, 153 b, 153 c and 153 d are mounted on the die mountingregions 152 a, 152 b, 152 c and 152 d of the submount 151, respectively.For example, the first and the first broadband LED chip 153 a may emitlight in a substantially similar color range (C1) as a second LED chip153 d (C1). A third LED chip 153 c may be larger or smaller than otherchips to emit a different amount and/or character of light (C2). Afourth chip LED chip 153 c may be shaped differently to emit a thirdcolor intensity and/or character of light (C3). It is to be understoodthat combinations of the three colors C1, C2 and C3 of light may becombined to provide white light or another desired color distribution.

FIGS. 2A-2C illustrate broadband LED chips and corresponding energy banddiagrams according to some embodiments of the present invention.Referring now to FIG. 2A, a first LED chip 203 a includes an indiumgallium nitride (InGaN) based active region 205 a provided between ap-type cladding layer 208 a and n-type cladding layer 209 a. TheInGaN-based active region 205 a is a multi-quantum well structureincluding a plurality of alternating active layers 206 a, 206 a′, and206 a″ and barrier layers 207 a, 207 a′, 207 a″, and 207 a′″. In someembodiments, the LED chip 203 a may correspond to the LED chip 103 a ofFIG. 1A.

The active layers 206 a, 206 a′, and 206 a″ each include differentrelative concentrations of indium (In) and gallium (Ga) selected toprovide a broadband blue LED. For example, the active layer 206 maycomprise In_(x)Ga_(1-x)N, where the average indium concentration x maybe in the range 0.12≦x≦0.19, corresponding to light from about 440 nm to500 nm. More particularly, the active layer 206 a may compriseIn_(x)Ga_(1-x)N in which the average indium concentration x is about0.13, the active layer 206 a′ may comprise In_(y)Ga_(1-y)N, in which theaverage indium concentration y is about 0.15, and the active layer 206a″ may comprise Ga_(z)N_(1-z), in which the average indium concentrationz is about 0.17. The indium gallium nitride active layers 206 a, 206 a′,and 206 a″ may have equal and/or different thicknesses, for example,between about 1 nm and 100 nm. In some embodiments, the indium and/orgallium concentration may also be varied over the thickness of one ormore of the active layers 206 a, 206 a′, and 206 a″ to provide stepwiseand/or continuous grading within one or more layers. The relativeconcentrations and/or thicknesses of the indium gallium nitride activelayers 206 a, 206 a′, and 206 a″ are selected such that the LED chip 203a emits light of a plurality of different wavelengths over a bluewavelength range (e.g., about 440 nm to about 500 nm). It is to beunderstood that the indium concentrations provided in the above exampleare approximate and illustrative, and as such, may be adjusted in eachof the quantum wells to obtain a desired concentration.

More particularly, as illustrated in the energy band diagram of FIG. 2A,the multi-quantum well structure 205 a of the LED chip 203 a includesInGaN active quantum well layers 206 a, 206 a′, and 206 a″ of varyingstoichiometries. The energy levels of the conduction band edge 211 a andvalence band edge 212 a are shown schematically; the levels areassociated with the material used to produce the barrier layers 207 a,207 a′, 207 a″, and 207 a′″. For the broadband blue LED 203 a, thebarrier layers 207 a, 207 a′, 207 a″, and 207 a′″ are formed from GaN.The relative concentrations of indium and gallium in the quantum wellactive layers 206 a, 206 a′, and 206 a″ are selected to define aplurality of different bandgap energies. As such, electrons and holes inthe quantum wells active layers 206 a, 206 a′, and 206 a″ recombine andemit light with energy consistent with the different bandgaps defined bythe quantum wells 206 a, 206 a′, and 206 a″ (E1 _(blue), E2 _(blue), E3_(blue), respectively). The different bandgaps provides light atdifferent average emission wavelengths λ_(1a), λ_(2a), and λ_(3a) withinthe blue wavelength range, which additively combine to provide anintegrated, broadband blue output light 215 a. The relativeconcentrations of indium and gallium in the quantum well layers 206 a,206 a′, and 206 a″ may be selected to provide the output light 215 aover a wavelength range of greater than about 30 mm with a centerwavelength of about 465 nm. It is to be understood that, while aparticular quantum well active layer may be formed with a target indiumconcentration to emit light at a target emission wavelength, the indiumin the quantum well is not homogeneous, and as such, there may be somevariation in the bandgap and hence the energy of the emitted light.Further, other physical processes such as thermal smearing, scattering,etc. may cause light to be emitted at not only the target wavelength,but also at wavelengths near the target wavelength.

The nonradiative energy loss E_(loss, blue) associated with producingthe output light 215 a is represented by the difference between theinput energy E_(g, blue) (defined by the energy required to raiseelectrons from the valence band 212 a into the conduction band 211 a)and the output energy E_(out, blue). E_(out, blue) is a function of thebandgap energies E1 _(blue), E2 _(blue), and E3 _(blue), and in somecases may be an average. For example, a broadband blue LED chip 203 a,formed from a base semiconductor material GaN (bandgap approximately3.65 eV), according to some embodiments of the present invention, mayhave a nonradiative energy loss E_(loss, blue) of about 0.65 eV to about1.0 eV when the bandgap of the quantum wells ranges from about 3.0 eV toabout 2.65 eV. The LED may also possess additional layers or deviceelements/structures, not shown in the LED schematic of FIG. 2A, that maylead to additional energy loss beyond the nonradiative energy lossE_(loss, blue) defined above.

Still referring to FIG. 2A, the GaN barrier layers 207 a, 207 a′, 207a″, and 207 a′″ have (Al, In, Ga)N compositions selected to providelarger bandgaps than the active layers 206 a, 206 a′, and 206 a″.Although illustrated as each including the same relative concentrationsof (Al, In, Ga)N, one or more of the barrier layers 207 a, 207 a′, 207a″, and 207 a′″ may be provided with different stoichiometries in someembodiments, as will be discussed in detail below with reference to FIG.3A. In addition, although illustrated in FIG. 2A as havingstoichiometries selected to provide sequentially decreasing bandgapenergies (for example, for ease in fabrication), other stoichiometriesmay also be used for the active layers, or even within one or more ofthe active layers.

Referring now to FIG. 2B, a second LED chip 203 b includes an indiumgallium nitride (InGaN) based active region 205 b provided between ap-type cladding layer 208 b and n-type cladding layer 209 b. The activeregion 205 b is a multi-quantum well structure including a plurality ofalternating InGaN active layers 206 b, 206 b′, and 206 b″ and barrierlayers 207 b, 207 b′, 207 b″, and 207 b″. Accordingly, the InGaN-basedactive region 205 b has a narrower bandgap than the InGaN-based activeregion 205 a of the first LED chip 203 a. In some embodiments, the LEDchip 203 b may correspond to the LED chip 103 b of FIG. 1A.

The active layers 206 b, 206 b′, and 206 b″ each include differentrelative concentrations of indium and gallium selected to provide abroadband green LED. For example, the active layer 206 b may compriseIn_(x)Ga_(1-x)N, where the average indium concentration x is about 0.20,the active layer 206 b′ may comprise In_(y)Ga_(1-y)N, where the averageindium concentration y is about 0.22, and the active layer 206 b″ maycomprise In_(z)Ga_(1-z)N, where the average indium concentration z isabout 0.26. As such, the target average wavelength for active layer 206b is about 515 nm, the target average wavelength for active layer 206 b′is about 540 nm and the target average wavelength for active layer 206b″ is about 565 nm. The InGaN active layers 206 b, 206 b′, and 206 b″may have equal and/or different thicknesses, for example, between about1 nm and 100 nm. In some embodiments, the indium and galliumconcentration may be varied over the thickness of one or more of theactive layers 206 b, 206 b′, and 206 b″ to provide stepwise and/orcontinuous grading within one or more layers. The relativeconcentrations and/or thicknesses of the InGaN active layers 206 b, 206b′, and 206 b″ are selected such that the LED chip 203 b emits light ofa plurality of different wavelengths over a green wavelength range(e.g., about 495 nm to about 590 nm). It is to be understood that theindium concentrations provided in the above example are approximate andillustrative, and as such, may be adjusted in each of the quantum wellsto obtain a desired concentration and/or emission characteristics.

More particularly, as illustrated in the energy band diagram of FIG. 2B,the multi-quantum well structure 205 b of the LED chip 203 b includesInGaN active quantum well layers 206 b, 206 b′, and 206 b″ of varyingstoichiometries selected to define a plurality of different bandgapenergies, such that the energy of the recombination across the differentquantum well layers 206 b, 206 b′, and 206 b″ (E1 _(green), E2 _(green),E3 _(green), respectively) provides light at different average emissionwavelengths λ_(1b), λ_(2b), and λ_(3b) within the green wavelengthrange. The different emission wavelengths λ_(1b), λ_(2b), and λ_(3b)additively combine to provide an integrated broadband green output light215 b. The relative concentrations of indium and gallium in the quantumwell layers 206 b, 206 b′, and 206 b″ may be selected to provide theoutput light 215 b over a wavelength range of greater than about 30 nm,with a center wavelength of about 540 nm.

The nonradiative energy loss E_(loss, green) associated with producingthe output light 215 b is represented by the difference between theinput energy E_(g, green) (which may be defined by the minimum energyrequired to raise an electron from the valence band 212 b into theconduction band 211 b) and the output photon energy E_(out, green).E_(out, green) is a function of the bandgap energies E1 _(green), E2_(green) and E3 _(green) (referred to collectively as E_(out, green)).The nonradiative energy loss for green emission is given byE_(loss, green)≈E_(g, green)−E_(out, green) (because of system effects,such as heating, the two sides of the equation may be approximatelyequal (≈)). For example, the nonradiative energy loss of a broadbandgreen LED chip 203 b, formed from a base semiconductor material GaN(having a bandgap of approximately 3.65 eV), according to someembodiments of the present invention, will be about 0.95 eV to about1.20 eV when the bandgap of the quantum wells ranges from 2.40 eV to2.90 eV. When the green broadband LED 203 b is formed from InGaN basematerial, instead of GaN or AlGaN, the overall nonradiative energy lossE_(loss, green) may be reduced relative to the overall nonradiativeenergy loss E_(loss, green) of the blue broadband LED 203 a. In otherwords, the nonradiative energy loss E_(loss, green) may be reduced byselecting a narrower-bandgap base material for the green LED chip 205 bto reduce the difference between the input and output energies. Forexample, if the LED is formed from a base material ofIn_(0.12)Ga_(0.88)N (having a bandgap of approximately 2.81 eV), the nonradiative energy loss is approximately 0.41 eV to 0.62 eV for lightemitter between 515 nm and 565 nm. It should be noted that, while lightemitted from a quantum well is referred to herein as having a specific,characteristic energy, such as E1 _(green) or E3 _(blue), this energy isan average energy, and thus the light emitted from each quantum well mayhave a distribution of energies scattered about the average value. Therange of emission energies may arise from thermal broadening and/or fromvariations in the well stoichiometries. Accordingly, greater energyefficiency may be achieved by using a narrower bandgap base materialand/or stoichiometries for the active layers 206 b, 206 b′, and 206 b″of the second LED chip 203 b than those of the first LED chip 203 a.

Still referring to FIG. 2B, the InGaN barrier layers 207 b, 207 b′, 207b″, and 207 b′″ have indium and gallium compositions selected to providelarger bandgaps than the active layers 206 b, 206 b′, and 206 b″.Although illustrated as including the same relative concentrations ofindium and gallium in FIG. 2B, one or more barrier layers 207 b, 207 b′,207 b″, and 207 b′″ may be provided with different relative compositionsof indium and gallium in some embodiments, as will be discussed indetail below with reference to FIG. 3B. Further, the stoichiometry ineach layer may vary. Also, although illustrated as havingstoichiometries to provide InGaN active layers 206 b, 206 b′, and 206 b″of sequentially decreasing bandgap energies, other elements and/orstoichiometries may also be used for the active layers.

Referring now to FIG. 2C, a third LED chip 203 c includes an aluminumgallium indium phosphide (AlGaInP) based active region 205 c providedbetween a p-type cladding layer 208 c and n-type cladding layer 209 c.The AlGaInP based active region 205 c is a multi-quantum well structureincluding a plurality of alternating AlGaInP active layers 206 c, 206c′, and 206 c″ and barrier layers 207 c, 207 c′, 207 c″, and 207 c′″.Accordingly, the AlGaInP based active region 205 c has a narrowerbandgap than the InGaN based active region 205 b of the second LED chip203 b. In some embodiments, the LED chip 203 c may correspond to the LEDchip 103 c of FIG. 1A.

The active layers 206 c, 206 c′, and 206 c″ each include differentrelative concentrations of aluminum, gallium, and/or indium configuredto provide a broadband red LED. For example, the active layer 206 c maycomprise Al_(x)Ga_(y)In_(1-x-y)P, targeting an average emissionwavelength of about 625 nm. The active layer 206 c′ may compriseAl_(w)Ga_(z)In_(1-w-z)P, targeting an average emission wavelength ofabout 650 nm. The active layer 206 c″ may compriseAl_(u)Ga_(v)In_(1-u-v)P, targeting an average emission wavelength ofabout 680 nm. The AlGaInP active layers 206 c, 206 c′, and 206 c″ mayhave equal and/or different thicknesses, for example, between about 1 nmand 100 nm. In some embodiments, the aluminum, gallium, and/or indium,concentrations may be varied over the thickness of one or more of theactive layers 206 c, 206 c′, and 206 c″ to provide stepwise and/orcontinuous grading within one or more layers. The relativeconcentrations and/or thicknesses of the AlGaInP active layers 206 c,206 e, and 206 c″ are selected such that the LED chip 203 c emits lightof a plurality of different wavelengths over a red wavelength range(e.g., about 600 nm to about 720 nm).

More particularly, as illustrated in the energy band diagram of FIG. 2C,the multi-quantum well structure 205 c of the LED chip 203 c includesAlGaInP active quantum well layers 206 c, 206 c′, and 206 c″, of varyingstoichiometries selected to define a plurality of different averagebandgap energies, such that the energy of the recombination across thedifferent bandgaps provides light at different average emissionwavelengths λ_(1c), λ_(2c), and λ_(3c) within the red wavelength range.The different average emission wavelengths λ_(1c), λ_(2c), and λ_(3c)additively combine to provide an integrated broadband red output light215 c. The relative concentrations of aluminum, gallium, and indium inthe quantum well layers 206 c, 206 c′, and 206 c″, as well as welldimensions may be selected to provide the output light 215 c over awavelength range of greater than about 30 nm with a center wavelength ofabout 665 nm.

The nonradiative energy loss N_(loss, red) associated with producing theoutput light 215 c is represented by the difference between the inputenergy E_(g, red) (defined by the energy of the AlGaInP barrier region207 c) and the photon energies consistent with the multi-quantum wellstructures with average band gap energies E1 _(red), E2 _(red) andE_(g, red) (referred to collectively as E_(out, red)). As noted above,the wavelength of an emitted photon is inversely proportional to thebandgap energy. Thus, the output energy for producing light in the redwavelength range E_(out, red) is less than the output energy forproducing light in the green and blue wavelength ranges E_(out, green)and E_(out, blue). However, because the bandgap energy E_(g, red) ofAlGaInP is narrower than the bandgap energies E_(out, green) andE_(out, blue) formed, for example, with GaN and InGaN, respectively, theoverall nonradiative energy loss E_(g, red) may be reduced. In otherwords, by selecting a narrower-bandgap material for the red LED chipactive region 205 c than that of the green LED chip active region 205 b,the difference between the input and output energies may be reduced,thereby reducing the nonradiative energy loss E_(loss, red).Accordingly, improved energy efficiency may be achieved by usingdifferent base materials having increasingly narrower bandgaps inbroadband LED chips that emit higher wavelength light.

Still referring to FIG. 2C, the AlGaInP barrier layers 207 c, 207 c′,207 c″, and 207 c′″ have aluminum, gallium, and indium compositionsselected to provide larger bandgaps than the active layers 206 c, 206c′, and 206 c″. Also, although illustrated in FIG. 2C as including thesame relative concentrations of aluminum, gallium, and indium, one ormore barrier layers 207 c, 207 c′, 207 c″, and 207 c′″ may be providedwith different relative compositions of aluminum, gallium, and/or indiumin some embodiments, as will be discussed in detail below with referenceto FIG. 3C. In addition, although illustrated as having stoichiometriesselected to provide sequentially decreasing bandgap energies, otherstoichiometries may also be used for the AlGaInP active layers.

Accordingly, the number, width, depth (based on stoichiometry),separation, doping, shape, and/or semiconductor material of one or moreof the wells shown in FIGS. 2A-2C may be modified to achieve a desiredspectral output. For example, the quantum well width may be modified byadjusting the quantum well growth time, changing the growth temperature,and/or adjusting the partial pressures of the chamber gases to selectaverage emission color or improve spectral distribution and/or alterefficiency. The well stoichiometries may also be adjusted by changingthe gas partial pressures and/or other growth parameters. Changes suchas these may be made during growth of the quantum well to produce wellswith nonuniform shape (i.e., varying stoichiometries). In addition, thesequence in which the quantum wells are formed may be selected based onthe requirements of the structure being grown. For example, a structurein including quantum wells having highly different stoichiometries maybe grown such that the concentration of a particular element issequentially increased in adjacent wells to provide quantum wells ofsequentially decreasing bandgaps, which may increase and/or maximizeefficiency and/or reduce reabsorption.

In addition, although illustrated in FIGS. 2A-2C as included in threeseparate chips 203 a, 203 b, and 203 c, it is to be understood that theGaN, InGaN, and/or AlGaInP multi-quantum well active regions 205 a, 205b, and/or 205 c may be formed on a common substrate. For example, aGaN-based multi-quantum well structure, an InGaN-based multi-quantumwell structure, and an AlGaInP-based multi-quantum well structure may beformed on a single substrate, and may respectively emit light over blue,green, and red wavelength ranges such that the combination is perceivedas white light.

FIGS. 3A-3C illustrate broadband LED chips and corresponding energy banddiagrams according to other embodiments of the present invention. In thebroadband LED chips of FIGS. 3A-3C, the stoichiometries of one or moreof the barrier layers and/or the quantum well active layers are alteredto aid performance.

More particularly, referring now to FIG. 3A, a blue LED chip 303 aincludes a gallium nitride (GaN) based active region 305 a between ap-type cladding layer 308 a and n-type cladding layer 309 a. The GaNbased active region 305 a is a multi-quantum well structure including aplurality of alternating InGaN active layers 306 a, 306 a′, and 306 a″and GaN barrier layers 307 a, 307 a′, 307 a″, and 307 a′″. In someembodiments, the LED chip 303 a may correspond to the LED chip 103 a ofFIG. 1A. In addition, the active layers 306 a, 306 a′, and 306 a″ may beconfigured similarly to the active layers 206 a, 206 a′, and 206 a″ ofthe LED chip 203 a of FIG. 2A to provide blue output light 315 a.However, the regions surrounding the quantum well active layers 306 a,306 a′, and 306 a″ have varying stoichiometries, and as such, are shapeddifferently relative to one another. In particular, the barrier layers307 a, 307 a′, 307 a″, and 307 a′″ have differing ratios of gallium tonitride that are selected to define a plurality of sequentiallydecreasing bandgap energies. The gallium concentrations may also bevaried over the thickness of the barrier layers 307 a′ and 307 a″between the active layers 306 a, 306 a′, and 306 a″ to provide stepwiseand/or continuous grading, while the barrier layers 307 a and 307 a′″adjacent the cladding layers 309 a and 308 a may have fixed galliumconcentrations. Thus, the relative concentrations of the barrier layers307 a, 307 a′, 307 a″, and 307 a′″ may be selected to guide and/orenhance recombination to provide improved efficiency in light emissionover a blue wavelength range (e.g., about 410 nm to about 495 nm).

Similarly, referring now to FIG. 3B, a green LED chip 303 b includes anindium gallium nitride (InGaN) based active region 305 b between ap-type cladding layer 308 b and n-type cladding layer 309 b. The InGaNbased active region 305 b is a multi-quantum well structure including aplurality of alternating InGaN active layers 306 b and 306 b′ and InGaNbarrier layers 307 b, 307 b′, and 307 b″. In some embodiments, the LEDchip 303 b may correspond to the LED chip 103 b of FIG. 1A. In addition,the active layers 306 b, and 306 b′ may be configured similarly to theactive layers 206 b and 206 b′ of the LED chip 203 b of FIG. 2B toprovide green output light 315 b. However, the barrier layers 307 b, 307b′, and 307 b″ have differing ratios of indium and gallium that areselected to define a plurality of sequentially decreasing bandgapenergies. In addition, the indium and/or gallium concentration may bevaried over the thickness of the barrier layer 307 b′ to providestepwise and/or continuous grading, while the barrier layers 307 b and307 b″ adjacent the cladding layers 309 b and 308 b may have fixedconcentrations. Thus, the relative concentrations of the barrier layers307 b, 307 b′, and 307 b″ may be selected to guide and/or enhancerecombination to provide improved efficiency in light emission over agreen wavelength range (e.g., about 495 nm to about 590 nm).

Likewise, referring now to FIG. 3C, a red LED chip 303 c includes analuminum gallium indium phosphide (AlGaInP) based active region 305 cbetween a p-type cladding layer 308 c and n-type cladding layer 309 c.The InGaN based active region 305 c is a multi-quantum well structureincluding a plurality of alternating AlGaInP active layers 306 c, 306c′, and 306 c″ and AlGaInP barrier layers 307 c, 307 c′, 307 c″, and 307c′″. In some embodiments, the LED chip 303 c may correspond to the LEDchip 103 c of FIG. 1A. However, the active layers 306 c, 306 c′, and 306c″ may have differing ratios of aluminum, gallium, and/or indium thatare selected to define a plurality of differing bandgap energies toprovide red output light 315 c. In particular, the aluminum, gallium,and/or indium concentrations may be varied over the thickness of theactive layers 306 c′ and 306 c″ while the active layer 306 c may have afixed concentration, to provide quantum well structures of differingshapes relative to one another. Likewise, the barrier layers 307 c, 307c′, 307 c″, and 307 c′″ have differing ratios of aluminum, gallium,and/or indium that are selected to define a plurality of differingbandgap energies. The aluminum, gallium, and/or indium concentrationsmay be varied over the thickness of the barrier layer 307 c″ to providestepwise and/or continuous grading, while the barrier layers 307 c, 307c′, and 307 e″ may have fixed concentrations. Thus, the relativeconcentrations of the barrier layers 307 c, 307 c′, 307 c″, and 307 c′″and/or the active layers 306 c, 306 c′, and 306 c″ may be selected toguide and/or enhance recombination to provide higher efficiency in lightemission over a red wavelength range (e.g., about 600 nm to about 720nm).

FIGS. 4A-4D are graphs illustrating example spectral emissioncharacteristics of light emitting device lamps according to someembodiments of the present invention. FIG. 4A illustrates an examplespectral output of a blue broadband LED chip according to someembodiments of the present invention, such as the LED chips 103 a, 203a, and 303 a of FIGS. 1A, 2A, and 3A. As shown in FIG. 4A, the lightemitted by the blue LED chip defines an asymmetric spectral distribution415 a over the blue wavelength range (e.g., about 410 nm-495 nm), as aresult of the combination of different narrowband emission wavelengths416 a provided by the active layers of the multi-quantum well bluebroadband LED chip. The spectral distribution 415 a is centered at awavelength (also referred to herein as a “center wavelength”) of about465 nm, while a peak wavelength 420 a of the spectral distribution 415 aoccurs toward an end of the blue wavelength range, for example, at about480 nm.

FIG. 4B illustrates an example spectral output of a green broadband LEDchip, according to some embodiments of the present invention, such asthe LED chips 103 b, 203 b, and 303 b of FIGS. 1A, 2B, and 3B. Referringnow to FIG. 4B, the light emitted by the green LED chip defines anasymmetric spectral distribution 415 b over the green wavelength range(e.g., about 495 nm-590 nm), resulting from the combination of differentnarrowband emission wavelengths 416 b provided by the active layers ofthe multi-quantum well green broadband LED chip. The spectraldistribution 415 b is centered at a wavelength of about 535 nm, while apeak wavelength 420 b of the spectral distribution 415 b occurs towardan end of the green wavelength range, for example, at about 560 nm.

FIG. 4C illustrates an example spectral output of a red broadband LEDchip according to some embodiments of the present invention, such as theLED chips 103 c, 203 c, and/or 303 c of FIGS. 1A, 2C, and 3C. As shownin FIG. 4C, the light emitted by the red LED chip defines an asymmetricspectral distribution 415 c over the red wavelength range (e.g., about600 nm-720 nm), as a result of the combination of different narrowbandemission wavelengths 416 c provided by the active layers of themulti-quantum well red broadband LED chip. The spectral distribution 415c is centered at a wavelength of about 665 nm, while a peak wavelength420 c of the spectral distribution 415 c occurs toward an end of the redwavelength range, for example, at about 690 nm.

FIG. 4D illustrates the combined spectral output of an LED lampincluding blue, green, and red broadband LED chips according to someembodiments of the present invention, such as the LED lamp 100 of FIG.1A. Referring now to FIG. 4D, the spectral distributions 415 a, 415 b,and 415 c of the blue, green, and red broadband LEDs combine to providean overall spectral distribution 400 that approximates the spectraldistribution of, for example, sunlight. However, as noted above, theshape of the emission spectra of the individual LEDs may be altered toprovide other desired spectral outputs by adjusting the stoichiometriesand/or material compositions of the active layers of the blue, green,and/or red broadband LED chips. In addition, the relative concentrationsof the active layers may be designed to produce an optimal spectrumunder particular operating conditions, taking into account, for example,the impact of thermal effects, increased current density, etc. Thecenter wavelengths of adjacent spectral distributions 415 a, 415 b, and415 c (shown in FIG. 4D at 465 nm, 535 nm, and 665 nm, respectively) maybe separated by less than a sum of the corresponding half width at halfmaximum values in some embodiments. In some embodiments, the energy (ornumber of photons) emitted at any wavelength within the spectraldistribution 400 may not exceed about 125% of the energy (or number ofphotons) emitted at another wavelength by any one of the blue, green, orred LED chips individually. Also, as LED lamps according to someembodiments of the present invention do not include light conversionmaterials, such as phosphors, operating efficiency may be improved.

FIGS. 5A and 5B illustrate LED lamps and corresponding spectral outputsaccording to further embodiments of the present invention. Referring nowto FIG. 5A, a multi-chip LED lamp 500 includes a common substrate orsubmount 501 including first and second die mounting regions 502 a and502 b. The die mounting regions 502 a and 502 b are each configured toaccept a broadband LED chip. The first and second broadband LED chips503 a and 503 b are mounted on the die mounting regions 502 a and 502 bof the submount 501, respectively. A light conversion material 506, suchas a phosphor, is configured to absorb at least some of the lightemitted by at least one of the broadband LED chips 503 a and 503 b andre-emit light of a different wavelength. In some embodiments, the lightconversion material 506 may be configured to emit light over awavelength range between that of the light emitted by the first andsecond broadband LED chips 503 a and 503 b. It is recognized that thelight conversion material 506 may differ in thickness or compositionacross the multi chip LED lamp 500. In one embodiment, LED 503 a mayexcite light conversion material 506 that is configured to absorb atwavelength λ_(absorb,B) and emit at λ_(emit,B) while LED 503 b mayexcite light conversion material 506 that is configured to absorb atwavelength λ_(absorb,R) and emit at λ_(emit,R). In this illustrativeexample, the distribution λ_(emit,B) may or may not overlap λ_(emit,R).

More particularly, as shown in FIG. 5A, the first broadband LED chip 503a is a blue LED chip configured to emit light in a blue wavelength range(i.e., between about 410-495 nm), and the second LED chip 503 b is a redLED chip configured to emit light in a red wavelength range (i.e.,between about 600-720 nm). For example, the first LED chip 503 a maycomprise a GaN multi-quantum well active region configured to providebroadband light output in the blue wavelength range, such as the LEDchip 203 a of FIG. 2A. Likewise, the second LED chip 503 b may comprisean AlGaInP multi-quantum well active region configured to providebroadband light output in the red wavelength range, such as the LED chip203 c of FIG. 2C.

Still referring to FIG. 5A, the light conversion material 506 is a greenphosphor, such as LuAG (Lanthanide+YAG), that is configured to absorb atleast some of the light emitted by the broadband LED chips 503 a and 503b and re-emit light in a green wavelength range (i.e., between about495-590 nm). The light conversion material 506 may be provided to atleast partially cover one or both of the LED chips 503 a and/or 503 busing many different techniques. For example, the light conversionmaterial 506 may be included in an encapsulant material in a plasticshell surrounding the LED chips 503 a and/or 503 b. In addition and/oralternatively, the light conversion material 506 may be directly coatedon the LED chips 503 a and/or 503 b, for example, as described in U.S.Patent Publication No. 2006/0063289, assigned to the assignee of thepresent invention. In other techniques, the light conversion material506 may be coated on the LED chips 503 a and/or 503 b using spincoating, molding, screen printing, evaporation and/or electrophoreticdeposition. Furthermore, the light conversion material 506 may beprovided by a semiconductor material, such as a direct bandgapsemiconductor. As such, the light emitted by the first and secondbroadband LED chips 503 a and 503 b and the light conversion material506 combines to provide white light.

FIG. 5B illustrates the combined spectral output of an LED lampincluding blue and red broadband LED chips and a green phosphoraccording to some embodiments of the present invention, such as the LEDlamp 500 of FIG. 5A. Referring now to FIG. 5B, the blue and redbroadband LED chips 503 a and 503 b respectively emit light havingcenter wavelengths of about 465 and 665 nm over a range of about 100 nm,as illustrated by spectral distributions 515 a and 515 b. As shown inFIG. 5B, the center wavelengths are separated by greater than the sum ofthe respective half width at half maximum values of the spectraldistributions 515 a and 515 b. The spectral distributions 515 a and 515b are asymmetric, with peak wavelengths occurring towards ends of theblue and red wavelength ranges, respectively. In addition, the lightconversion material 506 absorbs at least some of the light from the blueand red LED chips 503 a and 503 b and emits light having a centerwavelength of about 535 nm over a range of about 100 nm as shown byspectral distribution 515 c. The combination of the light emitted by theblue and red broadband LED chips 503 a and 503 b and the green lightconversion material 506 combines to provide an overall spectraldistribution 505, which is perceived as white light.

Although illustrated in FIGS. 5A-5B with reference to specificmaterials, different materials may be selected for the first and secondbroadband LED chips 503 a and 503 b and/or the light conversion material506 to provide relatively high-CRI white light output at a relativelyhigh efficiency. In addition, although illustrated as including twobroadband LED chips and a single light conversion material, additionalbroadband LEDs and/or light conversion materials may be included in LEDlamps according to some embodiments of the present invention to providea desired spectral output.

Accordingly, multi-chip lamps including a plurality of broadband LEDchips according to some embodiments of the present invention may beconfigured to provide high-CRI white light output with relatively highefficiency in comparison to conventional LED-based lamps. The broadbandLED chips discussed herein may be fabricated using, for example,epitaxial growth on suitable substrates using Metal Organic Vapor PhaseEpitaxy (MOVPE), Hydride Vapor Phase Epitaxy (HYPE), Molecular BeamEpitaxy (MBE), and/or other techniques. In addition, the LED chips maybe fabricated using patterning, etching, and/or dielectric and metaldeposition techniques and/or other techniques.

The size and design of the broadband LED chips described herein may beadjusted and/or optimized to provide a desired spectral output such as,for example, a number of photons emitted for a given current. Theindividual broadband LEDs may also be adjusted in size and/or design toprovide for a better match to the drive current source. Alternatively,each broadband LED in a multi-chip lamp according to some embodiments ofthe present invention may be excited with a separate current source sothat the relative light emitted may be adjusted and/or optimized toachieve a desired overall spectral output.

In the drawings and specification, there have been disclosed exemplaryembodiments of the invention. However, many variations and modificationscan be made to these embodiments without substantially departing fromthe principles of the present invention. Accordingly, although specificterms are used, they are used in a generic and descriptive sense onlyand not for purposes of limitation, the scope of the invention beingdefined by the following claims.

1. A light emitting device (LED), comprising: a broadband LED chipincluding a multi-quantum well active region comprising alternatingactive and barrier layers, the active layers respectively comprisingdifferent thicknesses and/or different relative concentrations of atleast two elements of a semiconductor compound and respectivelyconfigured to emit light of different emission wavelengths that definean asymmetric spectral distribution over a wavelength range within avisible spectrum.
 2. The device of claim 1, wherein the asymmetricspectral distribution of the light emitted by the active layers has afull width at half maximum (FWHM) of greater than about 35 nanometers(nm).
 3. The device of claim 1, wherein the light emitted by the activelayers combines to provide an appearance of white light.
 4. The deviceof claim 1, wherein the wavelength range comprises a broadband blue,green, or red wavelength range.
 5. The device of claim 4, wherein a peakwavelength of the spectral distribution is shifted toward an end of thewavelength range.
 6. The device of claim 5, wherein the peak wavelengthis within about 30 nanometers (nm) of the end of the wavelength range.7. The device of claim 4, wherein the wavelength range comprises theblue or green wavelength range, and wherein the active layers comprise afirst layer of In_(x)Ga_(1-x)N and a second layer of In_(y)Ga_(1-y)N,where x and y are not equal.
 8. The device of claim 4, wherein thewavelength range comprises the red wavelength range, and wherein theactive layers comprise a first layer of Al_(x)Ga_(y)In_(1-x-y)P and asecond layer of Al_(w)Ga_(z)In_(1-w-z)P, where x and w are not equal,and where y and z are not equal.
 9. The device of claim 1, furthercomprising: an n-type cladding layer on the multi-quantum well activeregion; and a p-type cladding layer on the multi-quantum well activeregion opposite the n-type cladding layer, wherein respective bandgapenergies of the active layers sequentially decrease from the n-typecladding layer to the p-type cladding layer.
 10. The device of claim 1,wherein at least one of the active layers has a non-uniformconcentration of the at least two elements over a thickness thereof. 11.The device of claim 1, wherein at least one of the barrier layerscomprises a graded concentration of the at least two elements of thesemiconductor compound based on the relative concentrations of theactive layers to enhance recombination of carriers therein.
 12. Thedevice of claim 1, wherein the wavelength range over which the activelayers are configured to emit light is greater than about 50 nanometers(nm).
 13. The device of claim 1, wherein the wavelength range over whichthe active layers are configured to emit light is less than about 100nanometers (nm).
 14. The device of claim 1, wherein an energy associatedwith the light emitted by one of the active layers is no more than about125% of an energy associated with the light emitted by another of theactive layers within the wavelength range.
 15. The device of claim 1,wherein the device is free of a wavelength conversion material.
 16. Thedevice of claim 1, further comprising: a second broadband LED chipincluding a multi-quantum well active region comprising alternatingsecond active and barrier layers, the second active layers respectivelycomprising different thicknesses and/or different relativeconcentrations of at least two elements of a second semiconductorcompound and respectively configured to emit light of different emissionwavelengths that define a second asymmetric spectral distribution over asecond wavelength range including wavelengths greater than those of thefirst wavelength range; and a third broadband LED chip including amulti-quantum well active region comprising alternating third active andbarrier layers, the third active layers respectively comprisingdifferent thicknesses and/or different relative concentrations of atleast two elements of a third semiconductor compound and respectivelyconfigured to emit light of different emission wavelengths that define athird asymmetric spectral distribution over a third wavelength rangeincluding wavelengths greater than those of the second wavelength range,wherein the third semiconductor compound has a narrower bandgap than thesecond semiconductor compound, wherein the second semiconductor compoundhas a narrower bandgap than the first semiconductor compound, andwherein the light emitted by the first, second, and third broadband LEDchips combines to provide an appearance of white light.
 17. The deviceof claim 16, wherein a separation between respective center wavelengthsof the first and second spectral distributions is not greater than a sumof respective half width at half maximum values of the first and secondspectral distributions, and wherein a separation between respectivecenter wavelengths of the second and third spectral distributions is notgreater than a sum of respective half width at half maximum values ofthe second and third spectral distributions.
 18. The device of claim 1,further comprising: a second broadband LED chip including amulti-quantum well active region comprising alternating second activeand barrier layers, the second active layers respectively comprisingdifferent thicknesses and/or different relative concentrations of atleast two elements of a second semiconductor compound and respectivelyconfigured to emit light of different emission wavelengths that define asecond asymmetric spectral distribution over a second wavelength rangeincluding wavelengths greater than those of the first wavelength range;and a wavelength conversion material configured to absorb at least someof the light emitted by the first and/or second broadband LED chips andre-emit light of different emission wavelengths over a third wavelengthrange, wherein the second semiconductor compound has a narrower bandgapthan the first semiconductor compound, and wherein the light emitted bythe first and second broadband LED chips and the wavelength conversionmaterial combines to provide an appearance of white light.
 19. A lightemitting device (LED), comprising: a broadband LED chip including amulti-quantum well active region comprising alternating active andbarrier layers of In_(x)Ga_(1-x)N where x≧0, the active layersrespectively comprising different thicknesses and/or different relativeconcentrations of indium (In) and gallium (Ga) and respectivelyconfigured to emit light of different emission wavelengths over a blueor green wavelength range to define a spectral distribution having afull width at half maximum (FWHM) of greater than about 40 nanometers(nm).
 20. The device of claim 19, wherein the spectral distribution isasymmetric over the blue or green wavelength range.